Breathing protection systems for the crew of aircraft likely to fly at high altitude include a regulator for feeding a breathing mask from a source of pressurized breathing gas (generally oxygen). The regulator can be carried by the mask or it can be mounted on the seat of the crew member.
Usually, such regulators include two selector members made available to the user:                a button for switching between normal and 100%, thereby enabling the mask to be fed with breathing gas that is diluted with air or else with pure gas; and        an “emergency” button which, when activated from a rest position, causes the mask to be fed at high pressure.        
The user thus has four possible operating states available:                1) normal, for use against insufficient oxygen;        2) 100%, rarely used, except for improving night vision;        3) normal in “emergency” mode, which should be avoided since the high pressure would give rise to a continuous leak through the air inlet; and        4) 100% in “emergency” mode for protecting the wearer against smoke and toxic gas by means of the high pressure which opposes ingress of air and/or depressurization of the environment at high altitude.        
The inventors have found that it suffices, in fact, to have only states 1 and 4 available, state 4 being able to replace state 2 without drawback, particularly since state 2 is little used.